Gallium Arsenide (GaAs) has been recognized as being very useful as the semiconductor base for very high speed VLSI circuits. The use of GaAs in integrated circuits has a number of advantages over conventional silicon technology. These include: High substrate bulk resistivity providing isolation and minimizes parasitic capacitance; Increased speed; Increased temperature tolerance; reduced power dissipation; and Improved radiation hardness.
GaAs is also useful as a photodetector and it has been known to produce photodetectors from GaAs which are analogous to existing silicon photodiodes.
Solid state imagers utilizing silicon technology are well known. Two kinds of imagers exist at the present time, these am charge coupled devices (CCD) and XY arrays. XY array is a generic term which includes charge injection devices (CID), mosaic arrays, transistor arrays and others. The charge coupled device is a somewhat esoteric and expensive imager, but offers good resolution. This technology is currently expanding the video camera market. The second type of imager is the XY array which consists of a matrix of transistors that can be addressed either by decoders or shift registers. It is simpler and cheaper but has lower resolution. It finds application in the industrial market where cosmetic image quality is not so important.
Although some attempts have been made to produce CCDs utilizing GaAs technology these are at an early stage. Due to the complicated nature of CCD devices and the cost of producing such a device, this technology is still formative. Also CCDs are more sensitive to defects than XY arrays and due to the higher defect density in GaAs, XY arrays offer some advantage.
Attempts have not been made to develop an XY imager based upon Gallium Arsenide. There are a number of reasons for this based upon perceived limitations of GaAs technology. Firstly, research has concentrated on the high speed processing aspects of GaAs MESFET IC technology whereas an XY imager is a low frequency device that, at first sight, would seem to be inappropriate for utilizing high speed technology. Secondly, it would seem that the lower doping of the GaAs substrate compared to silicon would result in a longer carrier diffusion path leading to increased crosstalk between imager pixels. Experiments performed by the inventors has proved that this is not the case. Thirdly, it was considered that the small voltage levels in GaAs would lead to a poor signal-to-noise ratio. However, this problem is not fundamental and the inventors have found that it can be overcome by an appropriate approach to circuit design. Furthermore, the inventors have recognized the advantage that GaAs ICe have a lower wiring capacitance than equivalent silicon devices, this leads to a lower kTC noise at the output and therefore a better than expected signal-to-noise ratio.
It is an intended object of this invention to provide a photoresponsive device based on standard Gallium Arsenide IC technology so that the imager can be integrated on the same substrate as the high speed digital processing elements. It is a further intended object to at least provide the public with a useful alternative to existing silicon devices.
An imaging device utilizing a GaAs digital IC process has the advantage that both the photoresponsive elements and the processing circuitry can be formed on the same substrate using very large scale integration (VLSi) techniques. This offers the opportunity of utilizing the high speed characteristics of GaAs for signal processing and the optical characteristics for imaging. This leads to cheaper and more compact imaging systems.
The inventors have also discovered that the optical gain of a GaAs MESFET sharply increases in the region where the gate crosses the transistor edge. This effect only occurs with a series gate resistor inserted, to produce the conditions for photovoltaic gate biasing. This effect occurs when the gate photocurrent follows through an external series gate resistor, R.sub.g, thus increasing the gate voltage and hence drain current. To produce a significant increase in drain current, a large R.sub.g introduces a large RC time constant which typically causes the response to roll-off in the 10-100 MHz range.
This edge effect is observed in a planar GaAs MESFET and is quite different to the edge gain effect observed in mesa GaAs MESFET structures. Photocollection under the gate overhang, outside the transistor, is suggested to explain the gain effect, in the planar device.
This new optical edge gain effect in planar GaAs MESFETs is useful in low frequency applications, such as in an imager. Thus a further object of this invention is to provide a GaAs optical imager utilizing this discovery.